[3.19.y-ckt stable] Patch "drm/dp/mst: Calculate MST PBN with 31.32 fixed point" has been added to the 3.19.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    drm/dp/mst: Calculate MST PBN with 31.32 fixed point

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt16.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 1789066abe6fd2d42a3513766c12e9cd1b7cd956 Mon Sep 17 00:00:00 2001
From: Harry Wentland <harry.wentland at amd.com>
Date: Fri, 22 Jan 2016 17:07:26 -0500
Subject: drm/dp/mst: Calculate MST PBN with 31.32 fixed point

commit a9ebb3e46c7ef6112c0da466ef0954673ad36832 upstream.

Our PBN value overflows the 20 bits integer part of the 20.12
fixed point. We need to use 31.32 fixed point to avoid this.

This happens with display clocks larger than 293122 (at 24 bpp),
which we see with the Sharp (and similar) 4k tiled displays.

Signed-off-by: Harry Wentland <harry.wentland at amd.com>
Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
Signed-off-by: Dave Airlie <airlied at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 67 ++++++++++++++++++++---------------
 1 file changed, 39 insertions(+), 28 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index dd63756..60e7ac6 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2552,32 +2552,31 @@ EXPORT_SYMBOL(drm_dp_check_act_status);
  */
 int drm_dp_calc_pbn_mode(int clock, int bpp)
 {
-	fixed20_12 pix_bw;
-	fixed20_12 fbpp;
-	fixed20_12 result;
-	fixed20_12 margin, tmp;
-	u32 res;
-
-	pix_bw.full = dfixed_const(clock);
-	fbpp.full = dfixed_const(bpp);
-	tmp.full = dfixed_const(8);
-	fbpp.full = dfixed_div(fbpp, tmp);
-
-	result.full = dfixed_mul(pix_bw, fbpp);
-	margin.full = dfixed_const(54);
-	tmp.full = dfixed_const(64);
-	margin.full = dfixed_div(margin, tmp);
-	result.full = dfixed_div(result, margin);
-
-	margin.full = dfixed_const(1006);
-	tmp.full = dfixed_const(1000);
-	margin.full = dfixed_div(margin, tmp);
-	result.full = dfixed_mul(result, margin);
-
-	result.full = dfixed_div(result, tmp);
-	result.full = dfixed_ceil(result);
-	res = dfixed_trunc(result);
-	return res;
+	u64 kbps;
+	s64 peak_kbps;
+	u32 numerator;
+	u32 denominator;
+
+	kbps = clock * bpp;
+
+	/*
+	 * margin 5300ppm + 300ppm ~ 0.6% as per spec, factor is 1.006
+	 * The unit of 54/64Mbytes/sec is an arbitrary unit chosen based on
+	 * common multiplier to render an integer PBN for all link rate/lane
+	 * counts combinations
+	 * calculate
+	 * peak_kbps *= (1006/1000)
+	 * peak_kbps *= (64/54)
+	 * peak_kbps *= 8    convert to bytes
+	 */
+
+	numerator = 64 * 1006;
+	denominator = 54 * 8 * 1000 * 1000;
+
+	kbps *= numerator;
+	peak_kbps = drm_fixp_from_fraction(kbps, denominator);
+
+	return drm_fixp2int_ceil(peak_kbps);
 }
 EXPORT_SYMBOL(drm_dp_calc_pbn_mode);

@@ -2585,11 +2584,23 @@ static int test_calc_pbn_mode(void)
 {
 	int ret;
 	ret = drm_dp_calc_pbn_mode(154000, 30);
-	if (ret != 689)
+	if (ret != 689) {
+		DRM_ERROR("PBN calculation test failed - clock %d, bpp %d, expected PBN %d, actual PBN %d.\n",
+				154000, 30, 689, ret);
 		return -EINVAL;
+	}
 	ret = drm_dp_calc_pbn_mode(234000, 30);
-	if (ret != 1047)
+	if (ret != 1047) {
+		DRM_ERROR("PBN calculation test failed - clock %d, bpp %d, expected PBN %d, actual PBN %d.\n",
+				234000, 30, 1047, ret);
 		return -EINVAL;
+	}
+	ret = drm_dp_calc_pbn_mode(297000, 24);
+	if (ret != 1063) {
+		DRM_ERROR("PBN calculation test failed - clock %d, bpp %d, expected PBN %d, actual PBN %d.\n",
+				297000, 24, 1063, ret);
+		return -EINVAL;
+	}
 	return 0;
 }

--
2.7.0